Since noble metals or alloys thereof are stable in electrical characteristics and highly resistant to corrosion and abrasion, they have been widely used, for example as materials for contacts of electronic components. Industrially, noble metals are chiefly used for electroplating, particularly gold electroplating, the gold plating involving high costs. High density surface mounting technology is the mainstream in the field of printed board production. In this technology, a surface mounted substrate is generally prepared by plating a copper circuit with gold. However, since gold plating diffuses with copper in soft soldering, the copper circuit must be pretreated with electroless nickel plating, which leads to complicated preparation process and lower productivity. In view of the above problems, the use of palladium electroplating in place of gold electroplating is on the increase.
However, the substitution of palladium for gold leaves another problem to be solved. Since a palladium plating formed according to the foregoing electroplating technology is not uniform in thickness, it comes to be inapplicable to recent high technology, e.g. more miniaturized and complicated electrical components. In contrast, according to electroless plating technology, a uniform plating layer can be deposited on electrical components having minute and complex configuration. Consequently, there have been proposed a lot of methods of electroless noble metal plating, particularly electroless palladium plating which has an advantage of lower costs over gold or platinum plating.
Compositions widely used for electroless palladium plating are, for example, a composition comprising a bivalent palladium salt, ammonia, ethylenediaminetetraacetate salt and hydrazine and a composition comprising a bivalent palladium salt, ethylenediamine, ethylenediaminetetraacetate salt and sodium hypophosphite (disclosed in Japanese Examined Patent Publication No. 26764/1971). However, these plating compositions are unstable in baths and decomposed in a short period of time. To improve the bath stability, methods comprising adding a bivalent sulfur-containing organic compound have been proposed in Japanese Examined Patent Publications No. 37045/1978 and Japanese Unexamined Patent Publication No. 124280/1987, etc.
However, according to the above organic compound-adding methods, considerably strong internal stress occurs in the plating layer deposited and is likely to cause cracks therein, which makes thick plating difficult or impossible. Under these circumstances, the development of an electroless palladium plating composition which is usable in an industrial scale is eagerly waited for in electronic industry and other fields.